Evaporation is a widely used separation technique in processing industries. Evaporation is used for instance to recover solvents in the plastics and metal finishing industries, particularly electroplating, in the pulp and paper industry to concentrate black liquor prior to incineration, and in the flavor and fragrance industries to concentrate raw products or recover valuable essences. In principle, evaporation involves concentration of a solution containing a non-volatile solute and a volatile solvent, usually water. Pure solvent is driven off and condensed, leaving an enriched concentrate. However, in many practical instances, the condensate from the evaporator also contains impurities that it is desirable to remove, either because they present a pollution problem preventing direct discharge of the condensate to the environment, or because they have economic value. For example, pulp mill water vapor condensates may contain 500 ppm or more methanol, as well as various oils and other organic substances. Both to reduce the pollution caused by discharging these streams directly, and to recover the methanol, it may be desirable to subject the condensate to a further treatment process. In application like this, where the condensate comprises an aqueous stream with a low concentration of organic contaminants, carbon adsorption is a widely used treatment process. A disadvantage of carbon adsorption, however, is the limited life of the activated carbon bed. Either the process must be interrupted while the bed is regenerated, or multiple beds in parallel must be available. Recovery of the adsorbed organics frequently involves the use of steam, or another solvent phase to desorb the organic, necessitating yet more treatment to separate this stream. Air stripping of the condensate stream is also possible, but creates odor and air pollution problems when the stream containing the stripped organic is discharged to the atmosphere. In the case of fruit juice processing, most fruit juices derive their flavor and aroma from organic components known as essences. When the fruit juice is concentrated, typically by multiple-effect evaporation, these essences are driven off with the evaporated water vapors. The essence-bearing evaporator condensate fractions from the various steps of the process are then separated by distillation to recover the concentrated essence. However, distillation is an energy-intensive, expensive process, especially with these streams that contain relatively small concentrations of organic constituents in large volumes of water. In the case of azeotropic mixtures, effective separation by distillation may not be commercially feasible. Another specific problem, particularly when handling citrus essences, is that the high temperatures involved in distillation may destroy the oily portion of the essence, with resulting loss of quality. From the above discussion it may be seen that in general, there is a need for an alternative improved separation process for handling evaporator condensates.
Pervaporation is a membrane separation process that has been applied to the separation of azeotropic mixtures. See, for example, U.S. Pat. Nos. 2,913,507, 2,953,502 and 2,981,680 to Binning et al. In particular, pervaporation can be used to dehydrate mixtures such as ethanol/water or ethyl acetate/water. Depending on the type of membrane used, the process may theoretically be water-selective or organic-selective, although to date the only available pervaporation systems use water-selective membranes. It is also known to separate hydrocarbon mixtures using pervaporation. For example, U.S. Pat. No. 3,930,990 to Brun et al. describes separation of butadiene from isobutene by pervaporation. U.S. Pat. No. 3,776,970, to Strazik et al. discloses a pervaporation process for separating styrene from ethylbenzene. U.S. Pat. No. 4,620,900, to Kimura e al., describes the use of a thermopervaporation process as an alternative to evaporation to remove water from solutions such as black liquor or orange juice.